Various converter topologies for reversible power flow are known from DE 40 26 955 C2. This deals with all possible kinds of energy converters, such as DC to DC converters, AC to DC converters, and AC to AC converters. Source and consumer are connected via semiconductor switches with an intermediate circuit with a storage element. Certain actuating principles and switching frequency periods are used to connect source and consumer alternately with the storage element. The storage element can be an HF transformer that galvanically isolates source and consumer.
Frequency converters enable a continuous setting of the speed of an asynchronous motor between zero and the rated speed with approximately constant torque. The actuating of other motors is also possible in theory. Frequency converters are used with preference, for example, in the field of climate control for regulating the air and volume flow of pumps, fans and compressors, in hoisting and conveying, and in the field of servo-drives.
Frequency converters can be adapted by their parameters to the particular motor being regulated. For example, a controllable-speed three-phase motor is known from DE 100 12 799 C2, which is designed to operate with a suitable frequency converter. The three-phase motor has a storage component as an “electronic nameplate”, in which the relevant motor data for the converter is kept. The converter can automatically read in the motor type as well as its data when placed in operation, so that the regulated parameters can be suitably chosen.
Various converter topologies are also known from wind power plants. For example, these are described by WANG, YANG, WU and HU in “Power Electronic Technology in Wind Generation System of Variable Speed—constant Frequency” (2009, 3rd International Conference on Power Electronics System and Application or in EP 1 921 738 A2.
A control circuit for a three-phase asynchronous motor is known from DE 10 2009 007 522 B4, in which each of the three mains phases is connected via a phase transistor to at least one of three stator windings of the three-phase asynchronous motor and via one freewheeling transistor each to a freewheeling collector potential. The stator windings are furthermore connected to the anode of one freewheeling diode each. The cathodes of the freewheeling diodes are connected to the freewheeling collector potential. The phase transistors can be switched on and off in any desired manner, without being disturbed by an induction voltage which is greater than the dielectric strength of the phase transistors.
DD 203 442 specifies a power converter system for control of several electrical servomotors with speed independent of each other. The power converter system comprises a frequency converter with a rectifier, a DC voltage intermediate circuit, an inverter and one or more medium-frequency transformers. The one or more medium-frequency transformers are arranged in the frequency converter after the inverter. Depending on the frequency being put out (0 to rated frequency of the motors), these transformers must have a considerable size, which makes them costly and limits the area of application. Furthermore, an externally commutated reversal control circuit is provided between transformer and motor. Due to the steep edges of the alternating voltage pulses at the output of the inverter and the stray capacitances of the motor arrangement, leakage currents arise on the motor bearings, whose outer races are conductively connected to the ground potential via the housing, which can result in destruction of the bearings in the medium term. To prevent this, partially isolated and thus expensive bearings are used in the motors. Owing to the high frequency of the stray leakage currents, however, the isolating effect is often not enough. Voltage peaks relating to the ground potential acting on the stator winding can also damage the motor on account of the high intermediate circuit voltage.
In DE 10 2005 016 962 A1 for the protecting of the bearings of the three-phase machine it is proposed to connect each of the three circuits of the stator isolated from each other to a separate connection point of the frequency converter and to design the frequency converter with a DC intermediate circuit grounded at the midpoint. Thanks to the interplay of the symmetrical stray capacitances and the grounding-symmetrical voltage, at every point in time a symmetrical electrical voltage—in terms of ground potential—should be present between the respective connection points for each circuit. This should prevent bearing currents caused by voltages that are capacitively coupled to the rotor.
Frequency converters with an integrated sine wave filter acting on all poles are known for a “soft operation” of motors. These frequency converters provide a sinusoidal output voltage, so that no bearing currents occur in the motor on account of steep voltage pulse edges. The frequency converter and the integrated sine wave filter must be adapted to the particular motor by the selection of operating modes. Each time the actuation of the power semiconductor is program controlled and modified for the particular application. Such all-pole sine wave filters have a complicate switching and are not suited for a simple connection of various motors.
Many of the frequency converters known from the prior art have to be especially adapted to the particular application in their parameters. At present, this is usually done by a keyboard/display unit, which is present at the converter and enables a navigation in the menu structure. Especially complex converters allow for a programming in a special programming language or through a corresponding graphics program on the PC. Readymade data sets are then loaded into the converter via an interface. It is also customary to save the ready parameter set on a storage medium (such as USB, chip and flash cards), which is inserted into a corresponding interface of the frequency converter and read in by it. Some models can themselves measure the drive characteristics and themselves set their own control parameters during setup (see Wikipedia: frequency converter).
DE 10 2005 042 319 A1 specifies a wide-range converter with a mains-side rectifier and several (n) load-side rectifiers. In the DC voltage intermediate circuit, a step-up/step-down adjusting device is provided with n transmitters, which couple the mains-side to the load-side rectifiers in electrical isolation. The load-side rectifiers are each individually power-optimized to a load. The mains-side and load-side rectifiers are mechanically moved away from each other.